Lower Saxony was founded as Land Niedersachsen on 1st November 1946 as a
merger of the former LänderBraunschweig [Brunswick], Oldenburg, Schaumburg-Lippe and Hannover [Hanover]. Due to the long tradition of own arms and flags in these former territories, it was especially difficult to find a common flag for the new Land Niedersachsen. As a first unofficial flag of Lower Saxony one can consider the red flag with the white horse used by the first Prime Minister (Ministerpräsident) Kopf at his office. There is no evidence that this was used in other situations; it was probably disbanded with the introduction of the present flag in 1951. A second unofficial flag for Lower Saxony came into existence around 1948. This was a red-white bicolor with the coat-of-arms of Lower Saxony.

Flagge des Ministerpräsidenten

~9:13 by M. Schmöger

Red flag with a white horse used by the first Prime Minister (Ministerpräsident) Kopf at his office (...), probably abandoned with the introduction of the present flag in 1951. [See the current prime minister's flag.]

M. Schmöger, 22 Sep 2000

The above image appears to have 9:13 proportions  is this correct or should it be 2:3? If the 9:13 proportions are right, are they based on observation or were they regulated? Also, is this the same flag described under Hanover 1945-1946? I am speculating, but maybe the Hanoverian Oberpräsident was Kopf himself and he "inherited" the flag when he became Lower Saxon Ministerpräsident.

Santiago Dotor, 25 Sep 2000

Right, Hinrich Kopf was Regierungspräsident and Oberpräsident of Hanover 1945-1946, and Ministerpräsident of Lower Saxony 1946-1955. I do not know the actual difference of Oberpräsident and Regierungspräsident. The flag is actually the same. The proportions are
according to the pictures in the cited literature (Rabbow 1999 and Schnath 1961). I do not know if they were legally prescribed.

An unofficial flag for Lower Saxony came into existence around 1948. This was a red-white bicolor with the coat-of-arms of Lower Saxony (gules, a horse rearing argent). There are only pictures showing the use of this flag in a vertical variant, as frequently in Germany. This flag was shown in at least two different variants in Bonn:

the first photograph shows it in front of the Museum König, where the Parlamentarische Rat (the constituent assembly) assembled (1st September 1948);

the second is on a photograph from the Bundesrat (the German 'Senate').

In both cases it is flanked by the flags of the other Länder. Both flags differ in the size, position and design of the coat-of-arms. I attach a reconstruction of the first variant.
Sources:
Kuhn 1991, p. 90 and Hormann and Plaschke 2006, p.194
M. Schmöger, 22 Sep 2000

This flag was only used for official purposes in Bonn and never as a flag for the citizens, so it should be designated only as state flag, not as civil flag.
M. Schmöger, 6 Oct 2000

This banner was probably replaced, because it might have been confused with the provincial flag of former Prussian province of Westphalia.
Klaus-Michael Schneider, 22 Apr 2014